Digital images can be generated directly from scenes by digital capture devices, such as digital still or video cameras, or by scanning an image captured on a photographic negative or slide film, or by various other means. Whatever the form of capture, most digital images are ultimately intended for display either by printed hardcopy, projection, or electronic viewing device. In order to provide the most pleasing display, it is necessary that the color and/or brightness of the displayed image be adjusted according to the subject matter of the scene.
With color photographic printers that make prints from film negatives, various methods for determining amounts of exposure have been known and practically employed. A well-known printing system in which the printing light source intensity is adjusted during red, green, and blue exposures to levels which normalize the resulting integrated transmittances to a near-neutral color balance, i.e., “gray,” is based on U.S. Pat. No. 2,571,697 issued to R. M. Evans on Oct. 16, 1951, titled “METHOD FOR CORRECTING PHOTOGRAPHIC COLOR PRINTS.” This printing system produces satisfactory results from a large majority of the negatives of a given type of film. It has also been known in the art to adjust the rate of correction for red, green, and blue exposures based on a linear combination of the red, green, and blue large area transmission densities (LATD) of the original to be printed. Since the above-described conventional printing systems are based on the integrated transmission measurements conducted over the whole area of the original, the prints obtained are not always satisfactory. For instance, if the background of the principal subject matter is primarily red (e.g., red curtain or furniture), green (e.g., green grass or foliage) or blue (e.g., blue sky or water), color correction based only on the aforesaid LATD system is unsatisfactory. This problem is known as “color failure.” Further, if the background of the principal subject matter has a particularly high or low brightness, the conventional correction based on the integrated transmission density does not give satisfactory results. For example, when the principal subject matter has been photographed with a back light or in a spotlight, conventional correction will give unsatisfactory results. This is known as “density failure” or “brightness failure.”
It has also been known in the prior art to determine the exposure in a color printer based on the measured densities of divided areas of color originals in which the entire area of the original is divided into upper and lower, right and left, and central and peripheral sections. The exposure is determined based on a combination of the LATD and the densities of the divided areas. In this system, the yield of satisfactory prints is somewhat raised. However, since the density of the principal subject matter is not accurately measured in this system, the correction is not always effected in the most desirable manner.
It is also known in the art that color failures can be substantially reduced by the use of the subject failure suppression technique described in the journal article “Modem Exposure Determination for Customizing Photofinishing Printer Response,”, E. Goll et al., Journal of Applied Photographic Engineering, Vol. 5, No. 2, 1979. For color negative film printing systems, it is further known that the performance of the subject failure suppression technique is improved by determination of an exposure-level-dependent gray estimate for a particular length of film as disclosed in U.S. Pat. No. 5,959,720 issued to Kwon et al. on Sep. 28, 1999, titled “METHOD FOR COLOR BALANCE DETERMINATION.”
Further, in looking at printed color photographs, it is well known that most people are concerned about the faces of the figures when present in the scene content. Therefore, in printers, it is desirable that the faces of the figures be printed in a good condition. An exposure controlled to obtain a good skin color and density can increase the yield of satisfactory prints.
It is known in the prior art, as in U.S. Pat. No. 4,203,671 issued to Takahashi et al. on May 20, 1980, titled “METHOD OF DETECTING FLESH COLOR IN COLOR ORIGINALS,” to print color originals based on the skin color areas when the originals contain over a certain number of points of skin color. In order to carry out this method, it is necessary first to detect skin color in the color original. Under the method of U.S. Pat. No. 4,203,671 (referenced above), a skin color area is defined as one whose red, green, and blue densities fall within an ellipse when plotted in a two-dimensional coordinate system or within an ellipsoid when plotted in a three-dimensional coordinate system, the axes of which represent the red, green, and blue densities or combinations of the densities of red, green, and blue. When the measured color is contained in the predetermined ellipse or ellipsoid, the color is assumed to be skin. The predetermined ellipse or ellipsoid is constructed by measuring the color attributes of identified skin points in a number of color negatives.
U.S. Pat. No. 5,781,276 issued to Zahn et al. on Jul. 14, 1998, titled “PRINTING OF COLOR FILM” also discloses a method for using points of skin color in determination of printing exposure amounts. This method also first requires the detection of points of skin color in the original, and also accomplishes this by determining whether a point falls within a predetermined color space. The predetermined color space is constructed by measuring the color compositions of identified skin points. The method relies on further logic to distinguish skin points from non-skin points.
U.S. Pat. No. 6,473,198 issued to Matama on Oct. 29, 2002, titled “IMAGE PROCESSING APPARATUS” describes an improved image processing apparatus comprising a device for receiving input image data from a source of image data supply; an image processing device for performing necessary image processing on the received input image data to produce output image data; an extracting device for extracting at least one specified portion of an image carried by the input image data; and a setting device for setting image processing conditions in accordance with the at least one specified portion extracted by the extracting device and the input image data, and the image processing device performs the image processing on the input image data in accordance with the image processing conditions set by the setting device. Matama discloses that the “specified image portion” may be exemplified by the center of an image, its periphery, a principal subject, objects other than the principal subject, the face of a human subject and other parts of the human subject. In addition, Matama discloses varying image processing depending on the size of a face. Furthermore, to perform image processing under different conditions in the face and other regions, a weighting coefficient as a magnification factor may be varied according to the region if the processing is an arithmetic operation. If the processing uses LUTs, a plane of weighting coefficients for the face region is provided and, at the same time, different LUTs are provided for the face region (the extracted specified portion) and the other regions; the results of processing of the respective regions are multiplied by different weights (in the illustrated case, “weight” is used for the face region and “1-weight” for the other regions) and the multiplied results are thereafter combined. If the face and other regions are to have different tones, edges should not be visible; to this end, gradually varying weighting coefficients are, preferably, applied to the neighborhood of the outline of the face so that no discontinuity will occur at the outline of the face. It is noteworthy that the weighting function Matama teaches is related to how different operations in different parts should be blended together.
In co-pending application US20030035578 by Dupin et al. published Feb. 20, 2003 (filed Jul. 12, 2001), titled “METHOD FOR PROCESSING A DIGITAL IMAGE TO ADJUST BRIGHTNESS,” an initial scene balance algorithm is applied to the digital image to produce an initially scene balanced digital image, skin-colored pixels in the initial scene balanced digital image are detected according to a pre-determined skin probability density function, and a brightness adjustment amount is calculated based on a statistic of the detected skin-colored pixels and applied to the initial scene balanced digital image to produce a processed digital image having improved overall brightness.
For a series of original images, these methods require further improvements to account for mistakes by the skin pixel detection method in differentiating true skin pixels from other subject matters that accidentally have skin colors.
There is a need therefore, for a more reliable method of identifying skin pixels and an associated method of adjusting the image brightness in response to the outcome of the more reliable skin pixel finding method, that contributes to more desirable quality in the final image.